Datasheet

343
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
27.6 Addressing the Flash During Self-Programming
The Z-pointer is used to address the SPM commands. The Z pointer consists of the Z-registers ZL and ZH in the
register file, and RAMPZ in the I/O space. The number of bits actually used is implementation dependent. Note
that the RAMPZ register is only implemented when the program space is larger than 64KB.
Since the Flash is organized in pages, the Program Counter can be treated as having two different sections.
One section, consisting of the least significant bits, is addressing the words within a page, while the most
significant bits are addressing the pages. This is shown in Figure 27-4 on page 343. Note that the Page Erase
and Page Write operations are addressed independently. Therefore it is of major importance that the Boot
Loader software addresses the same page in both the Page Erase and Page Write operation. Once a
programming operation is initiated, the address is latched and the Z-pointer can be used for other operations.
The (E)LPM instruction use the Z-pointer to store the address. Since this instruction addresses the Flash byte-
by-byte, also bit Z0 of the Z-pointer is used.
Figure 27-4. Addressing the Flash During SPM
(1)
Note: 1. The different variables used in Figure 27-4 are listed in Table 27-10 on page 350.
Bit2322212019181716
15 14 13 12 11 10 9 8
RAMPZ RAMPZ7 RAMPZ6 RAMPZ5 RAMPZ4 RAMPZ3 RAMPZ2 RAMPZ1 RAMPZ0
ZH (R31) Z15 Z14 Z13 Z12 Z11 Z10 Z9 Z8
ZL (R30) Z7 Z6 Z5 Z4 Z3 Z2 Z1 Z0
76543210
PROGRAM MEMORY
0123
Z - POINTER
BIT
0
ZPAGEMSB
WORD ADDRESS
WITHIN A PAGE
PAGE ADDRESS
WITHIN THE FLASH
ZPCMSB
INSTRUCTION WORD
PAGE
PCWORD[PAGEMSB:0]:
00
01
02
PAGEEND
PAGE
PCWORDPCPAGE
PCMSB PAGEMSB
PROGRAM COUNTER